1. Field of the Invention
This invention relates to molds for forming objects, especially glass or plastic articles, and more particularly relates to improved means for controlling heat extraction from forming surfaces of such molds.
2. Description of the Prior Art
At present many molds, for example, those used to produce pressed or blown glass or plastic articles, are air, steam or fluid cooled. Air cooling, usually by compressed or fan air, is relatively expensive, and often is capable of removing, frequently with improper distribution, only limited amounts of heat from hot surfaces of a mold for a given volume of air. Steam is also a comparatively inefficient coolant, largely because of its high initial temperature. Heretofore known methods of fluid cooling, although often more satisfactory than air or steam cooling in terms of cost and efficiency, can also be undesirable where heat flow rates are low because normal application extracts too much heat. When excessive cooling occurs because of excessive heat transfer from segments of a forming surface of a mold, their temperature can be lowered to a point where, for example, the glass or plastic article being formed cracks or is otherwise damaged. In this regard, air cooling is known to be generally more controllable at lower heat flow levels than fluid cooling; the supply thereof, for example, usually can be shut off, increased or decreased more easily and the air expelled as desired from a cooling system. However, air cooling systems ordinarily consume considerable electrical power and can also produce high noise levels, known to be detrimental to workers and their efficiency.
Numerous techniques have been suggested and used for fluid cooling of molds, including mist-type, and continuous and intermittent water flow systems. For example, U.S. Pat. No. 4,140,512 discloses a closed, water cooling system within a mold body. The system is said to be capable of pressurization to a desired extent, thereby to control the amount of heat removed from forming surfaces of a mold which contact glass being pressed. The apparatus described by this patent, like many others commonly used for the fluid cooling of molds, places fluid-contacting surfaces of the mold immediately opposite the surfaces in contact with the hot material being formed, so that heat transfer occurs directly through the mold wall and into the fluid. The fluid-contacting surfaces, in many cases maintained at the same temperature as the fluid itself, can therefore be 212 degrees F. or greater, depending on the boiling point of the particular fluid used and its pressure.* Since commonly-used mold wall materials are generally good heat conductors, and because of the limited amount of heat generated in a typical mold used for forming glass or plastics, the temperature of the interior of a mold wall seldom will be more than 70 degrees F. higher than that of the fluid-contacting surfaces of the same mold. Thus, the temperatures of segments of the hot forming surfaces of the mold are often of the order of 280 degrees F. when a water cooling system is tried. Ideally, the forming surfaces of certain segments of glass molds, in order to produce a quality glass article, should be within the range of 650-1000 degrees F. for most generally-used types of glass. One method of achieving such temperatures would be to increase the thickness of the mold wall to an extent which would provide the required heat transfer therethrough; its thickness would usually have to be increased, for most molds and glasses, by a factor of at least 10. In most cases it would be necessary also to vary the mold wall thickness from portion to portion thereof to enable differing amounts of heat transfer from various segments of the forming surface requiring different optimum temperatures; almost certainly, such a mold would be prohibitively costly to manufacture, heavy and difficult for workers to handle in service and may not be accommodated on many forming machines. FNT *If it were desired to use liquid water at 700 degrees F., a pressure of at least 3094 psi. would be required; mercury has been found to be well suited for use in cooling plungers of glass-forming apparatus.